A digital camera converts an analog image formed through a lens to digital signals through sampling based upon the Nyquist sampling theorem. The high-frequency component in a range equal to and above a Nyquist frequency is folded at the first Brillouin zone, i.e., a zone well known in solid-state physics and is recognized as a low-frequency component, resulting in the occurrence of moiré attributable to a phenomenon referred to as the aliasing effect. In addition, color moiré also occurs at a single-plate color image sensor with color filters arrayed therein, due to the variance among color filter densities and the phase difference among the color filters.
The phenomenon of color moiré including an erroneous estimate made during color interpolation is usually referred to as the term “color artifact”. While a color artifact attributable to aliasing may be caused by an erroneous estimate made in chrominance component interpolation during color interpolation processing, it is normally difficult to distinguish such color artifacts from actual present color, based upon the interpolation algorithm. In addition, an interpolation estimate error attributable to aliasing of the luminance component is often referred to as spurious resolution or false image structure. Such false image structure, too, cannot be easily distinguished from an actual image structure based upon the interpolation algorithm.
FIGS. 1A-1C present diagrams of the frequency resolution ranges, i.e., the first Brillouin zones, of signals sampled in a Bayer array, whereas FIG. 2 schematically illustrates how aliasing resulting in color artifacts and false image structure may occur in a frequency range close to the Nyquist frequency. FIG. 1A is a diagram of the first Brillouin zone corresponding to the R component signals sampled in the Bayer array, FIG. 1B is a diagram of the first Brillouin zone corresponding to the G component signals sampled in the Bayer array and FIG. 1C is a diagram of the first Brillouin zone corresponding to the B component signals sampled in the Bayer array. Examples of diagrams of the first Brillouin zones for another color filter array, such as the delta array, are included in patent reference 1 disclosing an application having been submitted by the inventor of the present invention.
Under normal circumstances, once the color filter array is determined, central points (hereafter referred to as polar points) at which color artifacts•false image structure manifest in the frequency space (k space) can be automatically ascertained. Such a polar point appears at a corner of the polygon defining a first Brillouin zone and also appears at a middle point of a line segment defining part of the polygonal shape in the case of a square lattice. A circular zone plate (CZP) image exactly corresponds to a diagram of the resolution range for the k space, and FIG. 3 presents an example of polar points of color artifacts that may manifest when an achromatic CZP image is captured with an image sensor assuming the Bayer array.
It is known in the related art that color artifacts can be suppressed through color interpolation processing executed on image signals having been filtered through an optical low pass filter (OLPF), before digitally imaging the high-frequency component, which is the root cause of aliasing. This concept is disclosed in, for instance, patent reference 2, patent reference 3 and patent reference 4. Namely, in the case of a square lattice, four-point split may be achieved at a given polar point through two light beam separations, i.e., a horizontal split in correspondence to one pixel and a vertical split in correspondence to one pixel, so as to cancel out the frequency component at the particular polar point until the value of exactly 0 can be assumed for the MTF. This processing may be referred to as 100% OLPF in the sense that an MTF dip of an extinction frequency band is created at a frequency position equivalent to 100% of the Nyquist frequency by shifting the light beam by an extent exactly matching a one-pixel pitch and, for convenience, the processing may be notated as “100% hv” as the filter processing is executed along two directions, i.e., along the longitudinal (vertical) direction and the lateral (horizontal) direction. A schematic graph (the dotted line indicated as the OLPF) of the 100% OLPF is also included in FIG. 2.
Patent reference 5, relating to a single lens reflex camera used with exchangeable lenses, discloses a method for reducing the adverse effects of an aberration due to the difference in the optical path length attributable to different thicknesses assumed at four-point split type optical low pass filters which are achieved in conjunction with two cameras having different pixel pitches, e.g., a camera equipped with a 7 μm/pixel image sensor and another equipped with a 5 μm/pixel image sensor, and assume beam separation widths substantially matching the respective pixel pitches.
The publication includes descriptions (paragraphs [0007] and [0226]), given in reference to FIG. 18, that it is a commonly practiced routine in optical low pass filter use to set the light beam separation width in a range close to the pixel pitch in order to minimize the extent of moiré. Namely, it expounds upon the concept that no matter how the pixel pitch changes over a range of an 8.92 μm/pixel pitch through a 4.88 μm/pixel pitch, as shown in FIG. 18, the frequency at the position assuming a constant ratio relative to the Nyquist frequency should be killed with regard to the light beam separation width.
Patent reference 1: Japanese Laid Open Patent Publication No. 2004-7164
Patent reference 2: Japanese examined utility model publication No. S47-18689
Patent reference 3: U.S. Pat. No. 4,626,897
Patent reference 4: U.S. Pat. No. 4,663,661
Patent reference 5: US Laid Open Patent Application No. 2005/0174467